Thursday, August 13, 2009

Mini-Splits or PTACS are the Key to Net Zero Energy Homes

High SEER minisplit heat pumps have finally caught the attention of high efficiency homebuilders. Postgreen, a redeveloper in Philadelphia, touts "geothermal efficiency at a fraction of the cost": http://www.postgreenhomes.com/models/
When you add PV solar to a house and a high efficiency and lo-temp-capable heat pump, then you really don't need to pipe natural gas to the home. (Cooking meals with gas may be preferred by chefs, but it is neither healthy nor energy efficient. Gas ranges put out a lot of CO). That saves the $12/month gas hookup fee.
Besides, you can't send natural gas back to the utility the way PV solar sends electricity back . In superinsulated homes, this scenario also makes solar thermal obsolete for heating and domestic hot water (DHW)
Why? Because even at a solar conversion efficiency of only 10%, in the summer, you are putting energy back on the grid and dollars in your pocket Every. Single. Day. Thus, the yearly system efficiency and ROI is better than solar thermal.
It's becoming accepted worldwide that the only way to acheive true net zero energy is with an all-electric house and PV. Minisplits are part of the puzzle.
A few more of benefits of these heat pumps:
1. Less floor space used inside the house for mechanicals.
2. Compression-cycle air conditioning included "for free".
3. Better zoning is possible than with any sort of central forced air system.
4. In smaller homes, the installed cost is a fraction of a centrally ducted system.

7 comments:

If you want to go gas-free but don't want to anger the chef in your life, you have to shell out some extra cash for an induction cook top. Since two of the three principles at Postgreen love to cook, we include one of these with all of our houses. They are pretty amazing and they're becoming the cooking standard in Europe. They are also twice as efficient as gas, and when you're pursuing net-zero energy on a budget, every bit of efficiency counts.

Note: PTAC's are the units that you see in motel rooms. The SEER ratings are nowhere near as good as the minisplits. I think it's mainly because the good minisplits are using high efficiency ECM motors. Mini PTAC's, however, can be found for as little as $500, and can be installed in an hour by unskilled labor. They are big enough to heat and cool an entire typical low energy house, so that's a very low price point! One more downside - they are usually noisier than minisplits, but in a low energy house, they never come on.

We built a superinsulated rental duplex w ptac heat pumps plus 400 watt baseboards in the bedrooms for heat (northern Indiana 6400 degree days) It works well and was cheap to install. The running cost savings to go to an efficient mini-split didn't justify the extra expense. See house at gregorylehman.com

Thanks for the information Greg. I'll check your site, but how does it handle outdoor temperatures of under 32F? Does it just go to resistance heat?

I think it's a very viable option for the low-energy-low-cost house, but you're the first person I've found to implement it. The average HVAC contractor would try to talk a builder out of it. How did you convince your guy?

We were able to do the installation of the PTAC's ourselves...just plug it in. The baseboards were wired by the electrician. I did the installation of the air-to-air heat exchanger and its ducts; also just plugs in.

The Amana heat pumps are rated down to 25 degrees F, after which its all resistance heat. We have a separate meter to track heating on one unit. 1200 sq ft: Feb 09 $45 and March 09 $27 for heat. We will get more numbers this winter.

About Me

He received a BS in Engineering Physics from the Colorado School of Mines in 1977. During the country's first Solar Era, he received an MS in Mechanical Engineering and Solar Technology from CSU (1979) After involvement in hundreds of solar thermal systems in the early 80's, he received several product design patents. The focus of this blog is low and zero energy home design and construction for Denver.